Traditionally, the spread or diffusion of a lighting fixture has been controlled by placing a lens, ground glass or other optical component in the path of light produced by the light source. The optical component may be made of glass, plastic or other suitable material. In order to control the amount of diffusion, the lens may be motorized and moved to different locations along the axis of the light path or moved relative to other optical components in the light path. Alternatively, a selection of lenses may be mounted on a wheel or semaphore arms to be placed into and removed from the light path.
Particularly where such lenses are positioned at the outlet, or mouth, of the fixture, their weight and the weight of mechanisms to move them may unbalance the head of the fixture. This imbalance may make an automated lighting fixture more difficult to move, causing overshoot when stopping or limiting the maximum speed at which the can be moved.
Lighting fixtures employing a parabolic or near-parabolic reflector emit a light beam comprised of substantially parallel light rays. As a result, when only a portion of the light beam emerging from the reflector is covered by a color filter, in an attempt to produce a light beam of variable saturation, some parts of the projected light beam are colored and the remainder is white. Similarly, when one portion of the light beam emerging from the reflector is covered by a first color filter and the remainder of the light beam is covered by a second color filter, in an attempt to produce a light beam of variable color, some parts of the projected light beam have the first color and the remaining parts have the second color.